Method and apparatus for controlling lean-burn engine to purge trap of stored NOx

ABSTRACT

A method and apparatus for controlling the operation of a “lean-burn” internal combustion engine in cooperation with an exhaust gas purification system having an emissions control device capable of alternatively storing and releasing NO x  when exposed to exhaust gases that are lean and rich of stoichiometry, respectively, determines a performance impact, such as a fuel-economy benefit, of operating the engine at a selected lean or rich operating condition. The method and apparatus then enable the selected operating condition as long as such enabled operation provides further performance benefits.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to methods and apparatus for controlling theoperation of “lean-burn” internal combustion engines used in motorvehicles to obtain improved engine and/or vehicle performance, such asimproved vehicle fuel economy or reduced overall vehicle emissions.

2. Background Art

The exhaust gas generated by a typical internal combustion engine, asmay be found in motor vehicles, includes a variety of constituent gases,including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides(NO_(x)) and oxygen (O₂). The respective rates at which an enginegenerates these constituent gases are typically dependent upon a varietyof factors, including such operating parameters as air-fuel ratio (λ),engine speed and load, engine temperature, ambient humidity, ignitiontiming (“spark”), and percentage exhaust gas recirculation (“EGR”). Theprior art often maps values for instantaneous engine-generated or“feedgas” constituents, such as HC, CO and NO_(x), based, for example,on detected values for instantaneous engine speed and engine load.

To limit the amount of engine-generated constituent gases, such as HC,CO and NOx, that are exhausted through the vehicle's tailpipe to theatmosphere as “emissions,” motor vehicles typically include an exhaustpurification system having an upstream and a downstream three-waycatalyst. The downstream three-way catalyst is often referred to as aNO_(x) “trap”. Both the upstream and downstream catalyst store NOx whenthe exhaust gases are “lean” of stoichiometry and release previouslystored No_(x) for reduction to harmless gases when the exhaust gases are“rich” of stoichiometry.

Significantly, in order to maximize the NO_(x)-storage capacity of thetrap, it is important to fully purge the trap of stored NO_(x). Theprior art teaches use of a “switching” oxygen sensor (HEGO) positioneddownstream of the trap, by which to detect, during a purge event, achange of the downstream exhaust gas from a near-stoichiometric air-fuelratio to a rich air-fuel ratio, at which point the trap is believed tobe “purged” of stored NO_(x). Unfortunately, because the downstreamexhaust gas is likely to go slightly rich of the stoichiometric air-fuelratio before the trap is completely purged of stored NO_(x), thetermination of a purge event based upon the switching of a downstreamHEGO sensor is likely to provide a premature indication of such a purge.An incomplete purge and, hence, a subsequent reduction of the actualNO_(x)-storage capacity of the trap, results.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and apparatus forcontrolling a lean-burn engine of a motor vehicle to more effectivelypurge an emissions control device of stored exhaust gas constituent,such as stored NO_(x), whereby lean-burn emissions control may beoptimized accessing the ability of a emissions control device for alean-burn engine to store an exhaust gas constituent.

In accordance with the invention, a method and apparatus are providedfor controlling the operation of an engine of a motor vehicle, whereinthe engine generates exhaust gas including an exhaust gas constituent,such as NO_(x), and wherein exhaust gas is directed through an emissionscontrol device before being exhausted to the atmosphere, whereupon thedevice stores a quantity of the exhaust gas constituent when the exhaustgas directed through the device is lean of stoichiometry and releasing apreviously-stored amount of the exhaust gas constituent when the exhaustgas directed through the device is rich of stoichiometry. Under theinvention, the method includes determining, during a rich, trap-purgingengine operating condition, a value related at least in part to thepresence of the exhaust gas constituent in the exhaust gas downstream ofthe device; and calculating the difference, if any, by which thedetermined value exceeds a predetermined value. The method furtherincludes accumulating the difference; and discontinuing the rich engineoperating condition when the accumulated difference exceeds apredetermined value.

In an exemplary method, the determination of the value of the firstexhaust gas constituent includes sampling the output signal generated bya first sensor having a sensitivity to the second exhaust gasconstituent. By way of example only, in the exemplary method, the firstexhaust gas constituent is NO_(x), and wherein the second exhaust gasconstituent is at least one of the group consisting of O₂, CO and HC.The exemplary method further includes determining a second value relatedat least in part to the presence of the second exhaust gas constituentupstream of the device, as through the use of a second, upstream sensor;and comparing the second value to the first value.

Other objects, features and advantages of the present invention arereadily apparent from the following detailed description of the bestmode for carrying out the invention when taken in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary system for practicing theinvention;

FIGS. 2-7 are flow charts depicting exemplary control methods used bythe exemplary system;

FIGS. 8A and 8B are related plots respectively illustrating a singleexemplary trap fill/purge cycle;

FIG. 9 is an enlarged view of the portion of the plot of FIG. 8Billustrated within circle 9 thereof;

FIG. 10 is a plot illustrating feedgas and tailpipe NO_(x) rates duringa trap-filling lean engine operating condition, for both dry andhigh-relative-humidity conditions; and

FIG. 11 is a flow chart depicting an exemplary method for determiningthe nominal oxygen storage capacity of the trap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an exemplary control system 10 for agasoline-powered internal combustion engine 12 of a motor vehicleincludes an electronic engine controller 14 having a processor (“CPU”);input/output ports; an electronic storage medium containingprocessor-executable instructions and calibration values, shown asread-only memory (“ROM”) in this particular example; random-accessmemory (“RAM”); “keep-alive” memory (“KAM”); and a data bus of anysuitable configuration. The controller 14 receives signals from avariety of sensors coupled to the engine 12 and/or the vehicle asdescribed more fully below and, in turn, controls the operation of eachof a set of fuel injectors 16, each of which is positioned to injectfuel into a respective cylinder 18 of the engine 12 in precisequantities as determined by the controller 14. The controller 14similarly controls the individual operation, i.e., timing, of thecurrent directed through each of a set of spark plugs 20 in a knownmanner.

The controller 14 also controls an electronic throttle 22 that regulatesthe mass flow of air into the engine 12. An air mass flow sensor 24,positioned at the air intake to the engine's intake manifold 26,provides a signal MAF representing the air mass flow resulting frompositioning of the engine's throttle 22. The air flow signal MAF fromthe air mass flow sensor 24 is utilized by the controller 14 tocalculate an air mass value AM which is indicative of a mass of airflowing per unit time into the engine's induction system.

A first oxygen sensor 28 coupled to the engine's exhaust manifolddetects the oxygen content of the exhaust gas generated by the engine 12and transmits a representative output signal to the controller 14. Thefirst oxygen sensor 28 provides feedback to the controller 14 forimproved control of the air-fuel ratio of the air-fuel mixture suppliedto the engine 12, particularly during operation of the engine 12 at ornear the stoichiometric air-fuel ratio (λ=1.00). A plurality of othersensors, indicated generally at 30, generate additional signalsincluding an engine speed signal N and an engine load signal LOAD in aknown manner, for use by the controller 14. It will be understood thatthe engine load sensor 30 can be of any suitable configuration,including, by way of example only, an intake manifold pressure sensor,an intake air mass sensor, or a throttle position/angle sensor.

An exhaust system 32 receives the exhaust gas generated upon combustionof the air-fuel mixture in each cylinder 18. The exhaust system 32includes a plurality of emissions control devices, specifically, anupstream threeway catalytic converter (“three-way catalyst 34”) and adownstream NO_(x), trap 36. The three-way catalyst 34 contains acatalyst material that chemically alters the exhaust gas in a knownmanner. The trap 36 alternately stores and releases amounts ofengine-generated NO_(x), based upon such factors, for example, as theintake air-fuel ratio, the trap temperature T (as determined by asuitable trap temperature sensor, not shown), the percentage exhaust gasrecirculation, the barometric pressure, the relative humidity of ambientair, the instantaneous trap “fullness,” the current extent of“reversible” sulfurization, and trap aging effects (due, for example, topermanent thermal aging, or to the “deep” diffusion of sulfur into thecore of the trap material which cannot subsequently be purged). A secondoxygen sensor 38, positioned immediately downstream of the three-waycatalyst 34, provides exhaust gas oxygen content information to thecontroller 14 in the form of an output signal SIGNAL0. The second oxygensensor's output signal SIGNAL0 is useful in optimizing the performanceof the three-way catalyst 34, and in characterizing the trap'sNO_(x)-storage ability in a manner to be described further below.

The exhaust system 32 further includes a NO_(x) sensor 40 positioneddownstream of the trap 36. In the exemplary embodiment, the NO_(x)sensor 40 generates two output signals, specifically, a first outputsignal SIGNAL1 that is representative of the instantaneous oxygenconcentration of the exhaust gas exiting the vehicle tailpipe 42, and asecond output signal SIGNAL2 representative of the instantaneous NO_(x)concentration in the tailpipe exhaust gas, as taught in U.S. Pat. No.5,953,907. It will be appreciated that any suitable sensor configurationcan be used, including the use of discrete tailpipe exhaust gas sensors,to thereby generate the two desired signals SIGNAL1 and SIGNAL2.

Generally, during vehicle operation, the controller 14 selects asuitable engine operating condition or operating mode characterized bycombustion of a “near-stoichiometric” air-fuel mixture, i.e., one whoseair-fuel ratio is either maintained substantially at, or alternatesgenerally about, the stoichiometric air-fuel ratio; or of an air-fuelmixture that is either “lean” or “rich” of the near-stoichiometricair-fuel mixture. A selection by the controller 14 of “lean burn” engineoperation, signified by the setting of a suitable lean-burn request flagLB_RUNNING_FLG to logical one, means that the controller 14 hasdetermined that conditions are suitable for enabling the system'slean-burn feature, whereupon the engine 12 is alternatingly operatedwith lean and rich air-fuel mixtures for the purpose of improvingoverall vehicle fuel economy. The controller 14 bases the selection of asuitable engine operating condition on a variety of factors, which mayinclude determined measures representative of instantaneous or averageengine speed/engine load, or of the current state or condition of thetrap (e.g., the trap's NO_(x)-storage efficiency, the current NO_(x)“fill” level, the current NO_(x) fill level relative to the trap'scurrent NO_(x)-storage capacity, the trap's temperature T, and/or thetrap's current level of sulfurization), or of other operatingparameters, including but not limited to a desired torque indicatorobtained from an accelerator pedal position sensor, the current vehicletailpipe NO_(x) emissions (determined, for example, from the secondoutput signal SIGNAL2 generated by the NO_(x) sensor 40), the percentexhaust gas recirculation, the barometric pressure, or the relativehumidity of ambient air.

Referring to FIG. 2, after the controller 14 has confirmed at step 210that the lean-burn feature is not disabled and, at step 212, thatlean-burn operation has otherwise been requested, the controller 14conditions enablement of the lean-burn feature, upon determining thattailpipe NO_(x) emissions as detected by the NO_(x) sensor 40 do notexceed permissible emissions levels. Specifically, after the controller14 confirms that a purge event has not just commenced (at step 214), forexample, by checking the current value of a suitable flag PRG_START_FLGstored in KAM, the controller 14 determines an accumulated measureTP_NOX_TOT representing the total tailpipe NO_(x) emissions (in grams)since the start of the immediately-prior NO_(x) purge or desulfurizationevent, based upon the second output signal SIGNAL2 generated by theNO_(x) sensor 40 and determined air mass value AM (at steps 216 and218). Because, in the exemplary system 10, both the current tailpipeemissions and the permissible emissions level are expressed in units ofgrams per vehicle-mile-traveled to thereby provide a more realisticmeasure of the emissions performance of the vehicle, in step 220, thecontroller 14 also determines a measure DIST_EFF_CUR representing theeffective cumulative distance “currently” traveled by the vehicle, thatis, traveled by the vehicle since the controller 14 last initiated aNO_(x) purge event.

While the current effective-distance-traveled measure DIST_EFF_CUR isdetermined in any suitable manner, in the exemplary system 10, thecontroller 14 generates the current effective-distance-traveled measureDIST_EFF_CUR at step 220 by accumulating detected or determined valuesfor instantaneous vehicle speed VS, as may itself be derived, forexample, from engine speed N and selected-transmission-gear information.Further, in the exemplary system 10, the controller 14 “clips” thedetected or determined vehicle speed at a minimum velocity VS_MIN, forexample, typically ranging from perhaps about 0.2 mph to about 0.3 mph(about 0.3 km/hr to about 0.5 km/hr), in order to include thecorresponding “effective” distance traveled, for purposes of emissions,when vehicle is traveling below that speed, or is at a stop. Mostpreferably, the minimum predetermined vehicle speed VS_MIN ischaracterized by a level of NOx emissions that is at least as great asthe levels of NOx emissions generated by the engine 12 when idling atstoichiometry.

At step 222, the controller 14 determines a modified emissions measureNOX_CUR as the total emissions measure TP_NOX_TOT divided by theeffective-distance-traveled measure DIST_EFF_CUR. As noted above, themodified emissions measure NOX_CUR is favorably expressed in units of“grams per mile.”

Because certain characteristics of current vehicle activity impactvehicle emissions, for example, generating increased levels of exhaustgas constituents upon experiencing an increase in either the frequencyand/or the magnitude of changes in engine output, the controller 14determines a measure ACTIVITY representing a current level of vehicleactivity (at step 224 of FIG. 2) and modifies a predetermined maximumemissions threshold NOX_MAX_STD (at step 226) based on the determinedactivity measure to thereby obtain a vehicle-activity-modifiedNO_(x)-per-mile threshold NOX_MAX which seeks to accommodate the impactof such vehicle activity.

While the vehicle activity measure ACTIVITY is determined at step 224 inany suitable manner based upon one or more measures of engine or vehicleoutput, including but not limited to a determined desired power, vehiclespeed VS, engine speed N, engine torque, wheel torque, or wheel power,in the exemplary system 10, the controller 14 generates the vehicleactivity measure ACTIVITY based upon a determination of instantaneousabsolute engine power Pe, as follows:Pe=TQ *N* k _(I),where TQ represents a detected or determined value for the engine'sabsolute torque output, N represents engine speed, and k_(I), is apredetermined constant representing the system's moment of inertia. Thecontroller 14 filters the determined values Pe over time, for example,using a high-pass filter G₁(s), where s is the Laplace operator known tothose skilled in the art, to produce a high-pass filtered engine powervalue HPe. After taking the absolute value AHPe of thehigh-pass-filtered engine power value HPe, the resulting absolute valueAHPe is low-pass-filtered with filter G₁(s) to obtain the desiredvehicle activity measure ACTIVITY.

Similarly, while the current permissible emissions lend NOX_MAX ismodified in any suitable manner to reflect current vehicle activity, inthe exemplary system 10, at step 226, the controller 14 determines acurrent permissible emissions level NOX_MAX as a predetermined functionf₅ of the predetermined maximum emissions threshold NOX_MAX_STD based onthe determined vehicle activity measure ACTIVITY. By way of exampleonly, in the exemplary system 10, the current permissible emissionslevel NOX_MAX typically varies between a minimum of about 20 percent ofthe predetermined maximum emissions threshold NOX_MAX_STD forrelatively-high vehicle activity levels (e.g., for many transients) to amaximum of about seventy percent of the predetermined maximum emissionsthreshold NOX_MAX_STD (the latter value providing a “safety factor”ensuring that actual vehicle emissions do not exceed the proscribedgovernment standard NOX_MAX_STD).

Referring again to FIG. 2, at step 228, the controller 14 determineswhether the modified emissions measure NOX_CUR as determined in step 222exceeds the maximum emissions level NOX_MAX as determined in step 226.If the modified emissions measure NOX_CUR does not exceed the currentmaximum emissions level NOX_MAX, the controller 14 remains free toselect a lean engine operating condition in accordance withthe exemplarysystem's lean-burn feature. If the modified emissions measure NOX_CURexceeds the current maximum emissions level NOX_MAX, the controller 14determines that the “fill” portion of a “complete” lean-burn fill/purgecycle has been completed, and the controller immediately initiates apurge event at step 230 by setting suitable purge event flags PRG_FLGand PRG_START_FLG to logical one.

If, at step 214 of FIG. 2, the controller 14 determines that a purgeevent has just been commenced, as by checking the current value for thepurge-start flag PRG_START_FLG, the controller 14 resets the previouslydetermined values TP_NOX_TOT and DIST_EFF_CUR for the total tailpipeNO_(x) and the effective distance traveled and the determined modifiedemissions measure NOX_CUR, along with other stored values FG_NOX_TOT andFG_NOX_TOT_MOD (to be discussed below), to zero at step 232. Thepurg-restart flag PRG_START_FLG is similarly reset to logic zero at thattime.

Refining generally to FIGS. 3-5, in the exemplary system 10, thecontroller 14 further conditions enablement of the lean-burn featureupon a determination of a positive performance impact or “benefit” ofsuch lean-burn operation over a suitable reference operating condition,for example, a near-stoichiometric operating condition at MBT. By way ofexample only, the exemplary system 10 uses a fuel efficiency measurecalculated for such lean-burn operation with reference to engineoperation at the near-stoichiometric operating condition and, morespecifically, a relative fuel efficiency or “fuel economy benefit”measure. Other suitable performance impacts for use with the exemplarysystem 10 include, without limitation, fuel usage, fuel savings perdistance traveled by the vehicle, engine efficiency, overall vehicletailpipe emissions, and vehicle drivability.

Indeed, the invention contemplates determination of a performance impactof operating the engine 12 and/or the vehicle's powertrain at any firstoperating mode relative to any second operating mode, and the differencebetween the first and second operating modes is not intended to belimited to the use of different air-fuel mixtures. Thus, the inventionis intended to be advantageously used to determine or characterize animpact of any system or operating condition that affects generatedtorque, such as, for example, comparing stratified lean operation versushomogeneous lean operation, or determining an effect of exhaust gasrecirculation (e.g., a fuel benefit can thus be associated with a givenEGR setting), or determining the effect of various degrees of retard ofa variable cam timing (“VCT”) system, or characterizing the effect ofoperating charge motion control valves (“CMCV,” an intake-charge swirlapproach, for use with both stratified and homogeneous lean engineoperation).

More specifically, the exemplary system 10, the controller 14 determinesthe performance impact of lean-burn operation relative to stoichiometricengine operation at MBT by calculating a torque ratio TR defined as theratio, for a given speed-load condition, of a determined indicatedtorque output at a selected air-fuel ratio to a determined indicatedtorque output at stoichiometric operation, as described further below.In one embodiment, the controller 14 determines the torque ratio TRbased upon stored values TQ_(i,j,k) for engine torque, mapped as afunction of engine speed N, engine load LOAD, and air-fuel ratio LAMBSE.

Alternatively, the invention contemplates use of absolute torque oracceleration information generated, for example, by a suitable torquemeter or accelerometer (not shown), with which to directly evaluate theimpact of, or to otherwise generate a measure representative of theimpact of, the first operating mode relative to the second operatingmode. While the invention contemplates use of any suitable torque meteror accelerometer to generate such absolute torque or accelerationinformation, suitable examples include a strain-gage torque meterpositioned on the powertrain's output shaft to detect brake torque, anda high-pulse-frequency Hall-effect acceleration sensor positioned on theengine's crankshaft. As a further alternative, the inventioncontemplates use, in determining the impact of the first operating moderelative to the second operating mode, of the above-described determinedmeasure Pe of absolute instantaneous engine power.

Where the difference between the two operating modes includes differentfuel flow rates, as when comparing a lean or rich operating mode to areference stoichiometric operating mode, the torque or power measure foreach operating mode is preferably normalized by a detected or determinedfuel flow rate. Similarly, if the difference between the two operatingmodes includes different or varying engine speed-load points, the torqueor power measure is either corrected (for example, by taking intoaccount the changed engine speed-load conditions) or normalized (forexample, by relating the absolute outputs to fuel flow rate, e.g., asrepresented by fuel pulse width) because such measures are related toengine speed and system moment of inertia.

It will be appreciated that the resulting torque or power measures canadvantageously be used as “on-line” measures of a performance impact.However, where there is a desire to improve signal quality, i.e., toreduce noise, absolute instantaneous power or normalized absoluteinstantaneous power can be integrated to obtain a relative measure ofwork performed in each operating mode. If the two modes arecharacterized by a change in engine speed-load points, then the relativework measure is corrected for thermal efficiency, values for which maybe conveniently stored in a ROM look-up table.

Returning to the exemplary system 10 and the flow chart appearing asFIG. 3, wherein the performance impact is a determined percentage fueleconomy benefit/loss associated with engine operation at a selected leanor rich “lean-burn” operating condition relative to a referencestoichiometric operating condition at MBT, the controller 14 firstdetermines at step 310 whether the lean-burn feature is enabled. If thelean-burn feature is enabled as, for example indicated by the lean-burnrunning flag LB_RUNNING_FLG being equal to logical one, the controller14 determines a first value TQ_LB at step 312 representing an indicatedtorque output for the engine when operating at the selected lean or richoperating condition, based on its selected air-fuel ratio LAMBSE and thedegrees DELTA_SPARK of retard from MBT of its selected ignition timing,and further normalized for fuel flow. At step 314, the controller 14determines a second value TQ_STOICH representing an indicated torqueoutput for the engine 12 when operating with a stoichiometric air-fuelratio at MBT, likewise normalized for fuel flow. At step 316, thecontroller 14 calculates the lean-burn torque ratio TR_LB by dividingthe first normalized torque value TQ_LB with the second normalizedtorque value TQ_STOICH.

At step 318 of FIG. 3, the controller 14 determines a value SAVINGSrepresentative of the cumulative fuel savings to be achieved byoperating at the selected lean operating condition relative to thereference stoichiometric operating condition, based upon the air massvalue AM, the current (lean or rich) lean-burn air-fuel ratio (LAMBSE)and the determined lean-burn torque ratio TR_LB, whereinSAVINGS=SAVINGS+(AM*LAMBSE*14.65*(1-TR_LB)).At step 320, the controller 14 determines a value DIST_ACT_CURrepresentative of the actual miles traveled by the vehicle since thestart of the last trap purge or desulfurization event. While the“current” actual distance value DIST_ACT_CUR is determined in anysuitable manner, in the exemplary system 10, the controller 14determines the current actual distance value DIST_ACT_CUR byaccumulating detected or determined instantaneous values VS for vehiclespeed.

Because the fuel economy benefit to be obtained using the lean-burnfeature is reduced by the “fuel penalty” of any associated trap purgeevent, in the exemplary system 10, the controller 14 determines the“current” value FE_BENEFIT_CUR for fuel economy benefit only once per“complete” lean-fill/rich-purge cycle, as determined at steps 228 and230 of FIG. 2. And, because the purge event's fuel penalty is directlyrelated to the preceding trap “fill,” the current fuel economy benefitvalue FE_BENEFIT_CUR is preferably determined at the moment that thepurge event is deemed to have just been completed. Thus, at step 322 ofFIG. 3, the controller 14 determines whether a purge event has just beencompleted following a complete trap fill/purge cycle and, if so,determines at step 324 a value FE_BENEFIT_CUR representing current fueleconomy benefit of lean-burn operation over the last complete fill/purgecycle.

At steps 326 and 328 of FIG. 3, current values FE_BENEFIT_CUR for fueleconomy benefit are averaged over the first j complete fill/purge cyclesimmediately following a trap decontaminating event, such as adesulfurization event, in order to obtain a value FE_BENEFIT_MAX_CURrepresenting the “current” maximum fuel economy benefit which is likelyto be achieved with lean-burn operation, given the then-current level of“permanent” trap sulfurization and aging. By way of example only, asillustrated in FIG. 4, maximum fuel economy benefit averaging isperformed by the controller 14 using a conventional low-pass filter atstep 410. In order to obtain a more robust value FE_BENEFIT_MAX for themaximum fuel economy benefit of lean-burn operation, in the exemplarysystem 10, the current value FE_BENEFIT_MAX_CUR is likewise filteredover j desulfurization events at steps 412, 414, 416 and 418.

Returning to FIG. 3, at step 330, the controller 14 similarly averagesthe current values FE_BENFIT_CUR for fuel economy benefit over the lastn trap fill/purge cycles to obtain an average value FE_BENEFIT_AVErepresenting the average fuel economy benefit being achieved by suchlean-burn operation and, hence, likely to be achieved with furtherlean-burn operation. By way of example only, in the exemplary system 10,the average fuel economy benefit value FE_BENEFIT_AVE is calculated bythe controller 14 at step 330 as a rolling average to thereby provide arelatively noise-insensitive “on-line” measure of the fuel economyperformance impact provided by such lean engine operation.

Because continued lean-burn operation periodically requires adesulfurization event, when a desulfurization event is identified asbeing in-progress at step 332 of FIG. 3, the controller 14 determines avalue FE_PENALTY at step 334 representing the fuel economy penaltyassociated with desulfurization. While the fuel economy penalty valueFE_PENALTY is determined in any suitable manner, an exemplary method fordetermining the fuel economy penalty value FE_PENALTY is illustrated inFIG. 5. Specifically, in step 510, the controller 14 updates a storedvalue DIST_ACT_DSX representing the actual distance that the vehicle hastraveled since the termination or “end” of the immediately-precedingdesulfurization event. Then, at step 512, the controller 14 determineswhether the desulfurization event running flag DSX_RUNNING_FLG is equalto logical one, thereby indicating that a desulfurization event is inprocess. While any suitable method is used for desulfurizing the trap36, in the exemplary system 10, the desulfurization event ischaracterized by operation of some of the engine's cylinders with a leanair-fuel mixture and other of the engine's cylinders 18 with a richair-fuel mixture, thereby generating exhaust gas with a slightly-richbias. At the step 514, the controller 14 then determines thecorresponding fuel-normalized torque values TQ_DSX_LEAN and TQ_DSX_RICH,as described above in connection with FIG. 3. At step 516, thecontroller 14 further determines the corresponding fuel-normalizedstoichiometric torque value TQ_STOICH and, at step 518, thecorresponding torque ratios TR_DSX_LEAN and TR_DSX_RICH.

The controller 14 then calculates a cumulative fuel economy penaltyvalue at step 520, as follows:PENALTY=PENALTY+(AM/2*LAMBSE*14.65*(1-TR _(—) DSX_LEAN))+(AM/2*LAMBSE*14.65*(1-TR _(—) DSX_RICH))Then, at step 522, the controller 14 sets a fuel economy penaltycalculation flag FE_PNLTY_CALC_FLG equal to logical one to therebyensure that the current desulfurization fuel economy penalty measureFE_PENALTY_CUR is determined immediately upon termination of theon-going desulfurization event.

If the controller 14 determines, at steps 512 and 524 of FIG. 5, that adesulfurization event has just been terminated, the controller 14 thendetermines the current value FE_PENALTY_CUR for the fuel economy penaltyassociated with the terminated desulfurization event at step 526,calculated as the cumulative fuel economy penalty value PENALTY dividedby the actual distance value DIST_ACT_DSX. In this way, the fuel economypenalty associated with a desulfurization event is spread over theactual distance that the vehicle has traveled since theimmediately-prior desulfurization event.

At step 528 of FIG. 5, the controller 14 calculates a rolling averagevalue FE_PENALTY of the last m current fuel economy penalty valuesFE_PENALTY_CUR to thereby provide a relatively-noise-insensitive measureof the fuel economy performance impact of such desulfurization events.By way of example only, the average negative performance impact or“penalty” of desulfurization typically ranges between about 0.3 percentto about 0.5 percent of the performance gain achieved through lean-burnoperation. At step 530, the controller 14 resets the fuel economypenalty calculation flag FE_PNLTY_CALC_FLG to zero, along with thepreviously determined (and summed) actual distance value DIST_ACT_DSXand the current fuel economy penalty value PENALTY, in anticipation forthe next desulfurization event.

Returning to FIG. 3, the controller 14 requests desulfurization eventonly if and when such an event is likely to generate a fuel economybenefit in ensuing lean-burn operation. More specifically, at step 332,the controller 14 determines whether the difference by which between themaximum potential fuel economy benefit FE_BENEFIT_MAX exceeds thecurrent fuel economy benefit FE_BENEFIT_CUR is itself greater than theaverage fuel economy penalty FE_PENALTY associated with desulfurization.If so, the controller 14 requests a desulfurization event by setting a asuitable flag SOX_FULL_FLG to logical one. Thus, it will be seenthat/the exemplary system 10 advantageously operates to schedule adesulfurization event whenever such an event would produce improved fueleconomy benefit, rather than deferring any such decontamination eventuntil contaminant levels within the trap 36 rise above a predeterminedlevel.

In the event that the controller 14 determines at step 332 that thedifference between the maximum fuel economy benefit value FE_BENEFIT MAXand the average fuel economy value FE_BENEFIT AVE is not greater thanthe fuel economy penalty FE_PENALTY associated with a decontaminationevent, the controller 14 proceeds to step 336 of FIG. 3, wherein thecontroller 14 determines whether the average fuel economy benefit valueFE_BENEFIT_AVE is greater than zero. If the average fuel economy benefitvalue is less than zero, and with the penalty as associated with anyneeded desulfurization event already having been determined at step 336as being greater than the likely improvement to be derived from suchdesulfurization, the controller 14 disables the lean-burn feature atstep 340 of FIG. 3. The controller 14 then resets the fuel savings valueSAVINGS and the current actual distance measure DIST_ACT_CUR to zero atstep 342.

Alternatively, the controller 14 schedules a desulfurization eventduring lean-burn operation when the trap's average efficiency η_(ave) isdeemed to have fallen below a predetermined minimum efficiency η_(min).While the average trap efficiency η_(ave) is determined in any suitablemanner, as seen in FIG. 6, the controller 14 periodically estimates thecurrent efficiency η_(cur) of the trap 36 during a lean engine operatingcondition which immediately follows a purge event. Specifically, at step610, the controller 14 estimates a value FG_NOX_CONC representing theNO_(x) concentration in the exhaust gas entering the trap 36, forexample, using stored values for engine feedgas NO_(x) that are mappedas a function of engine speed N and load LOAD for “dry” feedgas and,preferably, modified for average trap temperature T (as by multiplyingthe stored values by the temperature-based output of a modifier lookuptable, not shown). Preferably, the feedgas NO_(x) concentration valueFG_NOX_CONC is further modified to reflect the NO_(x)-reducing activityof the three-way catalyst 34 upstream of the trap 36, and other factorsinfluencing NO_(x) storage, such as trap temperature T, instantaneoustrap efficiency η_(inst), and estimated trap sulfation levels.

At step 612, the controller 14 calculates an instantaneous trapefficiency value η_(inst) as the feedgas NO_(x) concentration valueFG_NOX_CONC divided by the tailpipe NO_(x) concentration valueTP_NOX_CONC (previously determined at step 216 of FIG. 2). At step 614,the controller 14 accumulates the product of the feedgas NO_(x)concentration values FG_NOX_CONC times the current air mass values AM toobtain a measure FG_NOX_TOT representing the total amount of feedgasNO_(x) reaching the trap 36 since the start of the immediately-precedingpurge event. At step 616, the controller 14 determines a modified totalfeedgas NO_(x) measure FG_NOX_TOT_MOD by modifying the current valueFG_NOX_TOT_as a function of trap temperature T. After determining atstep 618 that a purge event has just begun following a completefill/purge cycle, at step 620, the controller 14 determines the currenttrap efficiency measure η_(cur) as difference between the modified totalfeedgas NO_(x) measure FG_NOX_TOT_MOD and the total tailpipe NO_(x)measure TP_NOX_TOT (determined at step 218 of FIG. 2), divided by themodified total feedgas NO_(x) measure FG_NOX_TOT_MOD.

At step 622, the controller 14 filters the current trap efficiencymeasure measure η_(cur) for example, by calculating the average trapefficiency measure η_(ave) as a rolling average of the last k values forthe current trap efficiency measure η_(cur) At step 624, the controller14 determines whether the average trap efficiency measure η_(ave) hasfallen below a minimum average efficiency threshold η_(min). the averagetrap efficiency measure η_(ave) has indeed fallen below the minimumaverage efficiency threshold η_(min), the controller 14 sets both thedesulfurization request flag SOX_FULL_FLG to logical one, at step 626 ofFIG. 6.

To the extent that the trap 36 must be purged of stored NO_(x) torejuvenate the trap 36 and thereby permit further lean-burn operation ascircumstances warrant, the controller 14 schedules a purge event whenthe modified emissions measure NOX_CUR, as determined in step 222 ofFIG. 2, exceeds the maximum emissions level NOX_MAX, as determined instep 226 of FIG. 2. Upon the scheduling of such a purge event, thecontroller 14 determines a suitable rich air-fuel ratio as a function ofcurrent engine operating conditions, e.g., sensed values for air massflow rate. By way of example, in the exemplary embodiment, thedetermined rich air-fuel ratio for purging the trap 36 of stored NO_(x)typically ranges from about 0.65 for “low-speed” operating conditions toperhaps 0.75 or more for “high-speed” operating conditions. Thecontroller 14 maintains the determined air-fuel ratio until apredetermined amount of CO and/or HC has “broken through” the trap 36,as indicated by the product of the first output signal SIGNAL1 generatedby the NO_(x) sensor 40 and the output signal AM generated by the massair flow sensor 24.

More specifically, as illustrated in the flow chart appearing as FIG. 7and the plots illustrated in FIGS. 8A, 8B and 9, during the purge event,after determining at step 710 that a purge event has been initiated, thecontroller 14 determines at step 712 whether the purge event has justbegun by checking the status of the purge-start flag PRG_START_FLG. Ifthe purge event has, in fact, just begun, the controller resets certainregisters (to be discussed individually below) to zero. The controller14 then determines a first excess fuel rate value XS_FUEL_RATE_HEGO atstep 716, by which the first output signal SIGNAL1is “rich” of a firstpredetermined, slightly-rich threshold λ_(ref) (the first thresholdλ_(ref) being exceeded shortly after a similarly-positioned HEGO sensorwould have “switched”). The controller 14 then determines a first excessfuel measure XS_FUEL_(—)1 as by summing the product of the first excessfuel rate value XS_FUEL_RATE_HEGO and the current output signal AMgenerated by the mass air flow sensor 24 (at step 718). The resultingfirst excess fuel measure XS_FUEL_(—)1, which represents the amount ofexcess fuel exiting the tailpipe 42 near the end of the purge event, isgraphically illustrated as the cross-hatched area REGION I in FIG. 9.When the controller 14 determines at step 720 that the first excess fuelmeasure XS_FUEL_(—)1 exceeds a predetermined excess fuel thresholdXS_FUEL_REF, the trap 36 is deemed to have been substantially “purged”of stored NO_(x), and the controller 14 discontinues the rich (purging)operating condition at step 722 by resetting the purge flag PRG_FLG tological zero. The controller 14 further initializes a post-purge-eventexcess fuel determination by setting a suitable flag XS_FUEL_(—)2_CALCto logical one.

Returning to steps 710 and 724 of FIG. 7, when the controller 14determines that the purge flag PRG_FLG is not equal to logical one and,further, that the post-purge-event excess fuel determination flagXS_FUEL_2_CALC is set to logical one, the controller 14 begins todetermine the amount of additional exces fuel already delivered to (andstill remaining in) the exhaust system 32 upstream of the trap 36 as ofthe time that the purge event is discontinued. Specifically, at step726, the controller 14 starts determining a second excess fuel measureXS_FUEL_(—)2 by summing the product of the differenceXS_FUEL_RATE_STOICH by which the first output signal SIGNAL1 is rich ofstoichiometry, and summing the product of the differenceXS_FUEL_RATE_STOICH and the mass air flow rate AM. The controller 14continues to sum the difference XS_FUEL_RATE_STOICH until the firstoutput signal SIGNAL1 from the NO_(x) sensor 40 indicates astoichiometric value, at step 730 of FIG. 7, at which point thecontroller 14 resets the post-purge-event excess fuel determination flagXS_FUEL_(—)2_CALC to logical zero. The resulting second excess fuelmeasure value XS_FUEL_(—)2, representing the amount of ecess fuelexiting the tailpipe 42 after the purge event is discontinued, isgraphically illustrated as the cross-hatched area REGION II in FIG. 9.Preferably, the second excess fuel value XS_FUEL_(—)2 in the KAM as afunction of engine speed and load, for subsequent use by the controller14 in optimizing the purge event.

The exemplary system 10 also periodically determines a measure NOX_CAPrepresenting the nominal NO_(x) storage capacity of the trap 36. Inaccordance with a first method, graphically illustrated in FIG. 10, thecontroller 14 compares the instantaneous trap efficiency η_(inst), asdetermined at step 612 of FIG. 6, to the predetermined referenceefficiency value η_(ref). While any appropriate reference efficiencyvalue η_(ref) is used, in the exemplary system 10, the referenceefficiency value η_(ref) is set to a value significantly greater thanthe minimum efficiency threshold η_(min). By way of example only, in theexemplary system 10, the reference efficiency value η_(ref) is set to avalue of about 0.65.

When the controller 14 first determines that the instantaneous trapefficiency η_(inst) has fallen below the reference efficiency valueη_(ref), the controller 14 immediately initiates a purge event, eventhough the current value for the modified tailpipe emissions measureNOX_CUR, as determined in step 222 of FIG. 2, likely has not yetexceeded the maximum emissions level NOX_MAX. Significantly, as seen inFIG. 10, because the instantaneous efficiency measure η_(inst)inherently reflects the impact of humidity on feedgas NO_(x) generation,the exemplary system 10 automatically adjusts the capacity-determining“short-fill” times t_(A) and t_(B) at which respective dry andrelatively-high-humidity engine operation exceed their respective“trigger” concentrations C_(A) and C_(B). The controller 14 thendetermines the first excess (purging) fuel value XS_FUEL_1 using theclosed-loop purge event optimizing process described above.

Because the purge event effects a release of both stored NO_(x) andstored oxygen from the trap 36, the controller 14 determines a currentNO_(x)-storage capacity measure NOX_CAP_CUR as the difference betweenthe determined first excess (purging) fuel value XS_FUEL_(—)1 and afiltered measure O2_CAP representing the nominal oxygen storage capacityof the trap 36. While the oxygen storage capacity measure O2_CAP isdetermined by the controller 14 in any suitable manner, in the exemplarysystem 10, the oxygen storage capacity measure O2_CAP is determined bythe controller 14 immediately after a complete-cycle purge event, asillustrated in FIG. 11.

Specifically, during lean-burn operation immediately following acomplete-cycle purge event, the controller 14 determines at step 1110whether the air-fuel ratio of the exhaust gas air-fuel mixture upstreamof the trap 36, as indicated by the output signal SIGNAL0 generated bythe upstream oxygen sensor 38, is lean of stoichiometry. The controller14 thereafter confirms, at step 1112, that the air mass value AM,representing the current air charge being inducted into the cylinders18, is less than a reference value AMref, thereby indicating arelatively-low space velocity under which certain time delays or lagsdue, for example, to the exhaust system piping fuel system arede-emphasized. The reference air mass value AM_(ref) is preferablyselected as a relative percentage of the maximum air mass value for theengine 12, itself typically expressed in terms of maximum air charge atSTP. In the exemplary system 10, the reference air mass value AM_(ref)is no greater than about twenty percent of the maximum air charge at STPand, most preferably, is no greater than about fifteen percent of themaximum air charge at STP.

If the controller 14 determines that the current air mass value is nogreater than the reference air mass value AM_(ref) at step 1114, thecontroller 14 determines whether the downstream exhaust gas is still atstoichiometry, using the first output signal SIGNAL1 generated by theNO_(x) sensor 40. If so, the trap 36 is still storing oxygen, and thecontroller 14 accumulates a measure O2_CAP_CUR representing the currentoxygen storage capacity of the trap 36 using either the oxygen contentsignal SIGNAL0 generated by the upstream oxygen sensor 38, asillustrated in step 1116 of FIG. 11, or, alternatively, from theinjector pulse-width, which provides a measure of the fuel injected intoeach cylinder 18, in combination with the current air mass value AM. Atstep 1118, the controller 14 sets a suitable flag O2_CALC_FLG to logicalone to indicate that an oxygen storage determination is on-going.

The current oxygen storage capacity measure O2_CAP_CUR is accumulateduntil the downstream oxygen content signal SIGNAL1 from the NO_(x)sensor 40 goes lean of stoichiometry, thereby indicating that the trap36 has effectively been saturated with oxygen. To the extent that eitherthe upstream oxygen content goes to stoichiometry orrich-of-stoichiometry (as determined at step 1110), or the current airmass value AM rises above the reference air mass value AM_(ref) (asdetermined at step 1112), before the downstream exhaust gas “goes lean”(as determined at step 1114), the accumulated measure O2_CAP_CUR and thedetermination flag O2_CALC_FLG are each reset to zero at step 1120. Inthis manner, only uninterrupted, relatively-low-space-velocity “oxygenfills” are included in any filtered value for the trap's oxygen storagecapacity.

To the extent that the controller 14 determines, at steps 1114 and 1122,that the downstream oxygen content has “gone lean” following a suitablerelatively-low-space-velocity oxygen fill, i.e., with the capacitydetermination flag 02_CALC_FLG equal to logical one, at step 1124, thecontroller 14 determines the filtered oxygen storage measure 02_CAPusing, for example, a rolling average of the last k current values02_CAP_CUR.

Returning to FIG. 10, because the purge event is triggered as a functionof the instantaneous trap efficiency measure η_(inst) and because theresulting current capacity measure NOX_CAP_CUR is directly related tothe amount of purge fuel needed to release the stored NO_(x) from thetrap 36 (illustrated as REGIONS III and IV on FIG. 10 corresponding todry and high-humidity conditions, respectively, less the amount of purgefuel attributed to release of stored oxygen), a relatively repeatablemeasure NOX_CAP_CUR is obtained which is likewise relatively immune tochanges in ambient humidity. The controller 14 then calculates thenominal NO_(x)-storage capacity measure NOX_CAP based upon the last mvalues for the current capacity measure NOX_CAP_CUR, for example,calculated as a rolling average value.

Alternatively, the controller 14 determines the current trap capacitymeasure NOX_CAP_CUR based on the difference between accumulated measuresrepresenting feedgas and tailpipe NO_(x) at the point in time when theinstantaneous trap efficiency η_(inst), first falls below the referenceefficiency threshold η_(ref). Specifically, at the moment theinstantaneous trap efficiency η_(inst) first falls below the referenceefficiency threshold η_(ref), the controller 14 determines the currenttrap capacity measure NOX_CAP_CUR as the difference between the modifiedtotal feedgas NO_(x) measure FG_NOX_TOT_MOD (determined at step 616 ofFIG. 6) and the total tailpipe NOX measure TP_NOX_TOT (determined atstep 218 of FIG. 2). Significantly, because the reference efficiencythreshold η_(ref) is preferably significantly greater than the minimumefficiency threshold η_(min) the controller 14 advantageously need notimmediately disable or discontinue lean engine operation whendetermining the current trap capacity measure NOX_CAP_CUR using thealternative method. It will also be appreciated that the oxygen storagecapacity measure O2_CAP, standing alone, is useful in characterizing theoverall performance or “ability ” of the NO_(x) trap to reduce vehicleemissions.

The controller 14 advantageously evaluates the likely continued vehicleemissions performance during lean engine operation as a function of oneof the trap efficiency measures η_(inst), η_(cur) or η_(ave), and thevehicle activity measure ACTIVITY. Specifically, if the controller 14determines that the vehicle's overall emissions performance would besubstantively improved by immediately purging the trap 36 of storedNO_(x), the controller 14 discontinues lean operation and initiates apurge event. In this manner, the controller 14 operates to discontinue alean engine operating condition, and initiates a purge event, before themodified emissions measure NOX_CUR exceeds the modified emissionsthreshold NOX_MAX. Similarly, to the extent that the controller 14 hasdisabled lean engine operation due, for example, to a low trap operatingtemperature, the controller 14 will delay the scheduling of any purgeevent until such time as the controller 14 has determined that leanengine operation may be beneficially resumed.

Significantly, because the controller 14 conditions lean engineoperation on a positive performance impact and emissions compliance,rather than merely as a function of NO_(x) stored in the trap 36, theexemplary system 10 is able to advantageously secure significant fueleconomy gains from such lean engine operation without compromisingvehicle emissions standards.

While an exemplary system and associated methods have been illustratedand described, it should be appreciated that the invention issusceptible of modification without departing from the spirit of theinvention or the scope of the subjoined claims.

1. A method for controlling the operation of an internal combustionengine in a motor vehicle, wherein the engine generates exhaust gasincluding at least a first and a second exhaust gas constituent, andwherein exhaust gas is directed through an emissions control devicebefore being exhausted to the atmosphere, the device storing at leastthe first exhaust gas constituent when the exhaust gas directed throughthe device is lean of stoichiometry and releasing previously-storedfirst exhaust gas constituent when the exhaust gas directed through thedevice is rich of stoichiometry, the method comprising: during a richengine operating condition, determining a value related at least in partto the presence of the exhaust gas constituent in the exhaust gasdownstream of the device; calculating the difference, if any, by whichthe determined value exceeds a predetermined value; accumulating thedifference; and discontinuing the rich engine operating condition whenthe accumulated difference exceeds a predetermined value.
 2. The methodof claim 1, wherein the first exhaust gas constituent is NO_(x), andwherein the second exhaust gas constituent is at least one of the groupconsisting of O₂, CO and HC.
 3. The method of claim 1, includingresetting the accumulated difference to zero upon discontinuing the richengine operating condition.
 4. The method of claim 1, whereindetermining the first value related at least in part to the presence ofthe second exhaust gas constituent includes sampling the output signalgenerated by a first sensor having a sensitivity to the second exhaustgas constituent.
 5. The method of claim 4, wherein the first sensor hasa sensitivity to both the first exhaust gas constituent and a secondexhaust gas constituent.
 6. The method of claim 4, further includingdetermining a second value related at least in part to the presence thesecond exhaust gas constituent upstream of the device; and comparing thesecond determined value to the first determined value.
 7. A method forcontrolling the operation of an internal combustion engine in a motorvehicle, wherein the engine generates exhaust gas including a firstexhaust gas constituent, and wherein exhaust gas is directed through anemissions control device before being exhausted to the atmosphere, thedevice storing a quantity of the first exhaust gas constituent when theexhaust gas directed through the device is lean of stoichiometry andreleasing a previously-stored amount of the first exhaust gasconstituent when the exhaust gas directed through the device is rich ofstoichiometry, the method comprising: during a rich engine operatingcondition, determining a first level of a second exhaust gas constituentin the exhaust gas downstream of the device; calculating the difference,if any, by which the determined level of the second exhaust gasconstituent exceeds a predetermined value, wherein the predeterminedvalue is slightly rich of a steoichiometric value; accumulating thedifference; and discontinuing the rich engine operating condition whenthe accumulated difference exceeds a predetermined value.
 8. The methodof claim 7, wherein the first exhaust gas constituent is NO_(x), andwherein the second exhaust gas constituent is at least one of the groupconsisting of O₂, CO and HC.
 9. The method of claim 7, includingresetting the accumulated difference to zero upon discontinuing the richengine operating condition.
 10. The method of claim 7, whereindetermining the first level of the second exhaust gas constituentincludes sampling the output signal generated by a first sensor having asensitivity to the second exhaust gas constituent.
 11. The method ofclaim 10, wherein the first sensor has a sensitivity to both the firstexhaust gas constituent and the second exhaust gas constituent.
 12. Themethod of claim 10, further including determining a second level of thesecond exhaust gas constituent upstream of the device; and comparing thesecond level to the first level.
 13. An apparatus controlling theoperation of an internal combustion engine in a motor vehicle, whereinthe engine generates exhaust gas including a first exhaust gasconstituent, and wherein exhaust gas is directed through an emissionscontrol device before being exhausted to the atmosphere, the devicestoring, quantity of the first exhaust gas constituent when the exhaustgas directed through the device is lean of stoichiometry and releasing apreviously-stored amount of the first exhaust gas constitiuent when theexhaust gas directed through the device is rich of stoichiometry, theapparatus comprising: a controller including a microprocessor arrangedto determine, during a rich engine operating condition, a first level ofa second exhaust gas constituent in the exhaust gas being exhausted tothe atmosphere, the controller being further arranged to calculate thedifference, if any, by which the determined first level of the secondexhaust gas constituent exceeds a first predetermined value, toaccumulate the difference over time, and to determine that substantiallyall of the previously-stored amount of the first exhaust gas constituenthas been released from the device when the accumulated differenceexceeds a second predetermined value.
 14. The apparatus of claim 13,wherein the first exhaust gas constituent is NO_(x), and wherein thesecond exhaust gas constituent is at least one of the group consistingof O₂, CO and HC.
 15. The apparatus of claim 13, wherein the controlleris further arranged to reset the accumulated difference to zero upondetermining that substantially all of the previously-stored amount ofthe first exhaust gas constituent has been released from the device. 16.The apparatus of claim 13, wherein the controller is further arranged tosample the output signal generated by a first sensor having asensitivity to the second exhaust gas constituent.
 17. The apparatus ofclaim 16, wherein the first sensor has a sensitivity to both the firstexhaust gas constituent and the second exhaust gas constituent.
 18. Theapparatus of claim 16, wherein the controller is further arranged todetermine a second level of the second exhaust gas constituent upstreamof the device, and to compare the second level to the first level.